The major objective is to investigate the development of mastery motivation in the first year of life. Three aspects of mastery were distinguished: Problem Solving, Practicing Sensorimotor Skills, and Effect Production. The relationships among these components of mastery were explored, and the origins of mastery and its continuity were studied. The sample includes 75 infants from middle income families. The methods employed were: measures of mastery motivation; observation of mother-infant and father-infant interaction in the home; Bayley Scales of Infant Development. The types and responsiveness of stimulation parents provide were found to vary with parent and the age and sex of the child. Behavior of the mothers and the fathers was associated with their infant's on attempts to master the environment. With regard to the role of the parents the development of mastery, fathers were found to be an important influence in spite of the small amount of time spent with their infants. The analyses of the transitions from one mastery behavior to another revealed and interesting pattern of stability and change across age. The strategy for mastering the tasks was similar at 6 and 12 months. Differences were found in the transitions to and from the social behaviors which suggest less dependency on social reinforcement as an element of motivation to master objects. Significant relationships were found between the frequency of looking accompanied with smiling at 6 and 12 months and developmental competence at 30 months. This suggests that looking and smiling may be marker of the infant's ability to process visual information. Significant differences in the frequency of the co-occurrences of smiling and mastery behaviors were also found. When these findings are combined with the changes in sequences of the mastery behavior, they indicate, that from 6 to 12 months, infants shift from external to internal sources of reinforcement to maintain their motivation to master the environment. Work continues on elucidating the relationship of affect to the development of mastery motivation, as well as on investigating sequences (transitional probabilities) of mastery behavior and their relationships to developmental status.